Pregnancy Due Date Calculator

Pregnancy Due Date Calculator — LMP, Ultrasound, IVF & Week-by-Week Calendar

Find your estimated due date and see exactly how far along you are. This free pregnancy calculator supports all major dating methods — last menstrual period (LMP) with irregular cycle adjustment, first-trimester ultrasound measurements, IVF embryo transfers (Day 3 and Day 5 blastocyst) — and generates a complete week-by-week pregnancy calendar with key appointment milestones.

Unlike basic due date calculators that assume every woman has a perfect 28-day cycle, this tool lets you adjust for your actual cycle length, so women with cycles of 21–45 days get a meaningfully more accurate estimate. Your data never leaves your browser.

How to Use the Pregnancy Due Date Calculator

Calculate your due date in three steps:

1. Choose your method — select "Last Period (LMP)" for the standard Naegele's Rule calculation, "Ultrasound (CRL)" if you have a first-trimester scan with gestational age, or "IVF" if you know your embryo transfer date and type.
2. Enter your dates — for LMP, input the first day of your last period and adjust the cycle slider if your cycle differs from 28 days; for ultrasound, enter the scan date plus gestational age in weeks and days; for IVF, enter the transfer date and select Day 3 embryo or Day 5 blastocyst.
3. Read your results instantly — gestational age in weeks and days, estimated due date, pregnancy progress percentage, baby size comparison, and a full calendar of milestones and recommended appointments appears automatically with no button to press.

Pregnancy Due Date Calculator Examples

Edge case — cycle adjustment matters most at the extremes. A woman with a 40-day cycle who uses the standard 28-day formula will be told her due date is 12 days earlier than it really is. After the first ultrasound, the scan date usually overrides the LMP-based estimate anyway, which is why entering your ultrasound result gives the most accurate dating.

Naegele's Rule — How Pregnancy Due Dates Are Calculated

Naegele's Rule is the standard clinical method for estimating the estimated due date (EDD). Developed by German obstetrician Franz Karl Naegele in the 19th century, it remains the baseline method used globally.

The formula: Add 280 days (40 weeks) to the first day of the last menstrual period. The reasoning: human pregnancy averages 266 days from conception (38 weeks). Since ovulation occurs roughly on day 14 of a 28-day cycle, adding 14 days gives 266 + 14 = 280 days from LMP.

Cycle length adjustment: Naegele's Rule assumes a 28-day cycle. For every day your cycle differs from 28, your due date shifts by one day. A 35-day cycle adds 7 days; a 21-day cycle subtracts 7 days. This calculator applies this adjustment automatically via the cycle length slider.

IVF calculation: For IVF pregnancies, the standard formula uses the embryo transfer date instead of LMP. A Day 5 blastocyst is already 5 days old at transfer, so: EDD = Transfer date + 261 days. A Day 3 embryo: EDD = Transfer date + 263 days.

Common Use Cases

• Regular LMP dating: Most OB-GYN appointments start by confirming the due date from LMP — this calculator matches that calculation exactly, including the cycle adjustment most online tools skip.
• IVF pregnancies: Women undergoing IVF need the transfer-date method. This calculator covers both Day 3 and Day 5 blastocyst transfers, which have different offsets (263 vs 261 days).
• Irregular cycles: Women with PCOS, perimenopause, or simply non-28-day cycles get a significantly more accurate estimate by adjusting the cycle slider rather than using the default.
• Post-ultrasound recalculation: After a first-trimester scan that shows a different gestational age than LMP suggests, enter the ultrasound data to get the updated due date your provider will use.
• Planning leave and milestones: The calendar of key appointments (Nuchal Translucency, morphology scan, GTT) and milestones (trimester ends, full term) helps plan work leave and prenatal care schedules.

Common Mistakes with Due Date Calculators

• Using LMP when cycle is irregular: Entering LMP without adjusting the cycle length leads to estimates that can be off by a week or more. If your cycle is not 28 days, always adjust the slider or use your ultrasound result.
• Confusing fetal age with gestational age: Gestational age counts from the first day of LMP — so at "4 weeks pregnant," the embryo is only about 2 weeks old. Ultrasounds always report gestational age, not fetal age.
• Expecting to deliver exactly on the due date: Only about 5% of babies are born on their exact due date. The normal delivery window is 37–42 weeks. The due date is a statistical midpoint, not a deadline.
• Not updating after ultrasound: If your first-trimester ultrasound differs from your LMP date by more than 7 days, ACOG guidelines recommend using the ultrasound date. Enter the ultrasound data in this calculator to match what your provider will record.

Frequently Asked Questions

How accurate is a pregnancy due date calculator?

LMP-based calculations have an accuracy of roughly ±2 weeks for regular 28-day cycles, and less for irregular cycles. A first-trimester ultrasound (7–13 weeks) is accurate to ±3–5 days and is the most reliable dating method available. Only about 5% of babies are born on their exact calculated due date — most arrive within a week either side.

What is Naegele's Rule and is it still used today?

Naegele's Rule adds 280 days to the first day of the last menstrual period to estimate the due date. It was developed in the 1800s and remains the standard starting point used by OB-GYN providers worldwide. While first-trimester ultrasound is more accurate, Naegele's Rule is the first estimate made at every initial prenatal visit, and it serves as the reference point against which ultrasound measurements are compared.

Does this calculator support IVF due date calculation?

Yes. For a Day 5 blastocyst transfer (the most common type in modern IVF), add 261 days to the transfer date. For a Day 3 embryo transfer, add 263 days. The difference exists because a Day 5 blastocyst is already 5 days old at the time of transfer, so 266 days from conception minus 5 equals 261 days from transfer.

When does the second trimester start and end?

The first trimester covers weeks 1–12 (completed). The second trimester runs from week 13 through week 26. The third trimester begins at week 27 and continues until delivery, with the estimated due date at week 40. These boundaries are used consistently by most providers and pregnancy apps, though some sources define them slightly differently.

What should I do after I calculate my due date?

Schedule your first prenatal appointment as soon as possible — ideally before 8 weeks of pregnancy. Your provider will confirm the gestational age, order initial blood work, and schedule the first-trimester screening ultrasound (Nuchal Translucency, typically 11–13 weeks). Do not use this calculator as a substitute for prenatal care; it provides estimates, not diagnoses.

Medical disclaimer: All results from this calculator are estimates based on standard clinical methods. This tool does not replace medical advice, prenatal care, or ultrasound examinations conducted by qualified healthcare professionals. Always consult your doctor, midwife, or obstetrician for personalized guidance.

Resources

• ACOG — Methods for Estimating the Due Date — Official American College of Obstetricians and Gynecologists committee opinion on due date estimation methods.
• WHO — Preterm Birth Fact Sheet — WHO definition of preterm birth (before 37 weeks) and full term, useful context for understanding the 37–42 week normal window.